The present invention relates to a gas diffusion backing for use in fuel cells, a process for fabricating the gas diffusion backing, membrane electrode assemblies containing the gas diffusion backing and fuel cells containing said membrane electrode assemblies.
Hydrogen and methanol fuel cells are of considerable importance in the search for new energy technologies, see for example, Ullmann""s Encyclopedia of Industrial Chemistry, 5th ed. Vol. 12A, pp. 55ff, VCH, New York, 1989. Fuel cells utilize the reaction of hydrogen and oxygen to produce electricity. Reaction may be a direct reaction between hydrogen and oxygen, or between a hydrocarbon and oxygen, such as in the so-called direct methanol fuel cell. In either case, water is an unavoidable by-product of reaction. Efficient operation of a fuel cell requires continuous simultaneous delivery of reactants to the catalyst layers or electrodes and, on the cathode side, removal of water from the neighborhood of the catalyst layer or electrode. In the case of fuel cells employing polymer electrolyte membranes (PEM) such as Nafion(copyright) ionomer membrane available from E. I. du Pont de Nemours and Company, there is the further complicating factor that the membrane must be kept wet for good fuel cell performance. Thus, in the case of fuel cells utilizing PEM, xe2x80x9cwater managementxe2x80x9d is a complex problem for which numerous solutions have been proposed in the art. See Yeager et al, U.S. Pat. No. 4,975,172 for an example of such cells.
It has become common practice to incorporate fluoropolymers in the catalyst layer and gas diffusion backing to impart a degree of hydrophobicity to otherwise hydrophilic structures, an example being the use of polytetrafluoroethylene (PTFE) or copolymers thereof with hexafluoropropylene or a perfluorovinyl ether. (See Blanchart, U.S. Pat. No. 4,447,505, Yeager, op. cit., and Serpico et al, U.S. Pat. No. 5,677,074.) More recently, Kumar et al, WO 0067336, have disclosed the use of amorphous fluoropolymers for treating carbon fiber papers and fabrics to achieve enhanced hydrophobicity.
On the anode side of a PEM fuel cell, there is a tendency for the membrane to dry out as a result of what is known in the art as proton-drag. On the cathode side, there is a tendency towards flooding of the cathode by both the by-product water and net transport of water across the membrane by proton-drag. Thus it is necessary to provide a means for transporting water to the anode and away from the cathode.
The problem is further complicated by the need to continuously supply reactant (or fuel) gases, or in certain cases liquids, to the electrodes. The presence of a layer of liquid water at the electrodes, and in the channels leading to them, serves as a barrier to the reactants, degrading fuel cell performance.
The art provides numerous schemes for providing a balance between the water transport requirements and the reactant transport requirements. Among these schemes are several involving employment of a carbon fiber fabric or paper which has been treated to alter its wettability. These carbon fiber or carbon paper structures are known in the fuel cell art as gas diffusion backings (GDBs), and that is the term employed herein to mean carbon fiber and carbon paper structures, both treated and untreated, that are suitable for employment as GDBs in fuel cells.
Taniguchi et al, U.S. Pat. No. 6,083,638, discloses a fibrous carbon substrate pre-treated with a fluororesin which is baked at 360xc2x0 C., followed by treatment with particulate dispersions of hydrophobic and hydrophilic polymer to form discrete channels which are hydrophobic and hydrophilic.
Isono et al, EP 1 063 717 A2, discloses a fibrous carbon substrate treated with a high temperature fluoropolymer in aqueous dispersion in such a manner as to exhibit a gradient in hydrophobicity in a direction normal to the direction of ion transport through the cell. The fibrous carbon substrate is further treated with a mixture layer comprising the same aqueous dispersion, and exhibiting a similar gradient in hydrophobicity. The entire structure is subject to heating to 380xc2x0 C. to coalesce the polymer.
Cipollni, WO 01/17050 A1, discloses a porous carbon body with increased wettability by water which is achieved by treating a carbon fabric or paper with a solution of a metallic oxide hydrates among others, particular SnO2.
Fredley, U.S. Pat. No. 5,998,058, discloses the porous carbon body of Cipollni, op. cit., which has been further treated so that some of the interstices between the fibers of the porous carbon body are coated with polytetrafluoroethylene, thereby creating an intermixed network of hydrophilic and hydrophobic channels.
Dufner et al, U.S. Pat. No. 6,024,848, disclose the porous carbon body of Fredley, op. cit., further treated by application of a contact bilayer in the form of a coating deposited on the carbon substrate where the coating is made up of a combination of a hydrophobic and a hydrophilic polymer, the hydrophobic polymer being a copolymer of tetrafluoroethylene and hexafluropropylene, and the hydrophilic polymer being a perfluorinated ionomer.
Dirven et al, U.S. Pat. No. 5,561,000, discloses a bilayer structure in which a fine pore layer consisting of PTFE and carbon is deposited by coating onto a PTFE-treated carbon paper or fabric.
Gorman et al, WO 00/54350, disclose a variation on the bilayer of Dirven et al, op. cit., wherein the coarse pore layer, or carbon paper or fabric, is treated to be hydrophilic, but fail to disclose any manner in which such hydrophilic character may actually be realized.
U.S. Pat. No. 5,620,807 (Dow) describes 2 layer structures comprised of a small pore region and a large pore region. The large pore region is oriented against the bipolar plate, the small pore region oriented against the catalyst layer. The large pore region consists of a porous carbon paper. The small pore region is film coated from solvents.
Porous films and coating comprising polyvinylidene fluoride are known. For example Gozdz et al. (U.S. Pat. No. 5,418,091) disclose porous PVDF homopolymer and copolymer containing solutions of lithium salts in aprotic solvents useful as separators in lithium batteries.
In the first aspect, the invention provides gas diffusion backing adapted for use in fuel cells consisting essentially of a porous first layer and a microporous second layer in electrically conductive contact therewith, said first layer consisting essentially of a porous carbonaceous paper or fabric comprising carbon fibers, wherein the carbon fibers comprise at least 50% by volume of the layer, said fibers being at least partially coated by a first fluorinated polymer disposed thereupon, and said second layer consisting essentially of a second fluorinated polymer having carbon particles intermixed therewith, the first and second fluorinated polymers (i) being the same or different, (ii) each being a melt processable polymer selected from the group consisting of
(a) amorphous polymers having a glass transition temperature (Tg) of less than about 250xc2x0 C.;
(b) crystalline or semi-crystalline polymers having a melting point of less than about 315xc2x0 C., more typically less than about 265xc2x0 C., and still more typically less than about 250xc2x0 C.; and
(c) mixtures thereof.
In the first aspect, the first and second fluorinated polymers may have a weight average molecular weight of less than 500,000 Daltons.
In the first aspect, the invention further provides a microporous layer applied from a composition comprising a second fluorinated polymer, carbon particles, a first component, e.g. a solvent, typically having a boiling point of less than about 100xc2x0 C., and a second component, typically having a boiling point of at least about 100xc2x0 C., wherein the second component has a boiling point greater than that of the first component. The high boiling component may be a plasticizer or a solvent.
In the first aspect, the invention further provides first and second fluorinated polymers selected from the group consisting of a fluorinated ionomer comprising at least 6 mole % of monomer units having a fluorinated pendant group with a terminal ionic group; a copolymer or terpolymer comprising polyvinylidene fluoride and hexafluoropropylene; and mixtures thereof.
In a second aspect, the invention provides a process for forming a gas diffusion backing comprising:
(W) contacting a porous carbonaceous paper or fabric comprising carbon fibers with a first fluorinated polymer to impregnate said first fluorinated polymer into the paper or fabric and at least partially coat said fibers, thereby forming a porous first layer containing at least 50% by volume of carbon fibers;
(X) applying a second layer to the first layer from a composition comprising a second fluorinated polymer, carbon particles, a first component, e.g. a solvent, typically having a boiling point of less than about 100xc2x0 C., and a second component, typically having a boiling point of at least about 100xc2x0 C., wherein the second component has a boiling point greater than that of the first component, and wherein the first and second fluorinated polymers are (i) the same or different, (ii) each comprises a melt processable polymer selected from the group consisting of
(a) amorphous polymers having a glass transition temperature (Tg) of less than about 250xc2x0 C.;
(b) crystalline or semi-crystalline polymers having a melting point of less than about 315xc2x0 C., more typically less than 265xc2x0 C., and still more typically less than about 250xc2x0 C.; and
(c) mixtures thereof;
(Y) drying the first and second layers after each of steps (W) and (X), or after completion of step (X)to remove the low boiling solvent; and
(Z) heating the first and second layers individually, or after they have been brought into contact, to form the gas diffusion backing having a microporous second layer in electrical contact with the first layer.
In the second aspect, the invention provides applying, in step (X), by coating or lamination.
In a third aspect, the invention provides a membrane electrode assembly comprising:
(a) a solid polymer electrolyte membrane;
(b) at least one electrode; and
(c) a gas diffusion backing consisting essentially of a porous first layer and a microporous second layer in electrically conductive contact therewith, said first layer consisting essentially of a porous carbonaceous paper or fabric comprising carbon fibers, wherein the carbon fibers comprise at least 50% by volume of the layer, said fibers being at least partially coated by a first fluorinated polymer disposed thereupon, and said second layer consisting essentially of a second fluorinated polymer having carbon particles intermixed therewith, the first and second fluorinated polymers (i) being the same or different, (ii) each being a melt processable polymer selected from the group consisting of
(a) amorphous polymers having a glass transition temperature (Tg) of less than about 250xc2x0 C.;
(b) crystalline or semi-crystalline polymers having a melting point of less than about 315xc2x0 C., more typically less than about 26xc2x0 C., and still more typically less than about 250xc2x0 C.; and
(c) mixtures thereof.
In a fourth aspect, the invention provides a fuel cell comprising membrane electrode assembly, wherein the membrane electrode assembly comprises:
(a) a solid polymer electrolyte membrane;
(b) at least one electrode; and
(c) a gas diffusion backing consisting essentially of a porous first layer and a microporous second layer in electrically conductive contact therewith, said first layer consisting essentially of a porous carbonaceous paper or fabric comprising carbon fibers, wherein the carbon fibers comprise at least about 50% by volume of the layer, said fibers being at least partially coated by a first fluorinated polymer disposed thereupon, and said second layer consisting essentially of a second fluorinated polymer having carbon particles intermixed therewith, the first and second fluorinated polymers (i) being the same or different, (ii) each being a melt processable polymer selected from the group consisting of
(a) amorphous polymers having a glass transition temperature (Tg) of less than about 250xc2x0 C.;
(b) crystalline or semi-crystalline polymers having a melting point of less than about 315xc2x0 C., more typically less than about 265xc2x0 C., and still more typically less than about 250xc2x0 C.; and
(c) mixtures thereof.